Both the Aftonbladet and Expressen tabloids reported on Thursday morning that the couple exchanged engagement rings in New York about a month ago, but kept the engagement secret.
The engagement was confirmed shortly after 9am by the bride-to-be's mother Queen Silvia.
"The princess is very happy and so are we. He is every mother-in-law's dream," she told news agency TT.
Shortly after 9am on Thursday, the Royal Court also published confirmation of the rumours on its website:
"The Marshal of the Realm is delighted to announce the engagement between Princess Madeleine and Mr. Christopher O'Neill.
"After His Majesty The King of Sweden had given his consent to the marriage between Princess Madeleine and Mr Christopher O'Neill, His Majesty requested the approval by the Swedish Government, in accordance with the procedures set out in the Swedish Constitution."
Journalist Ebba von Sydow, expert on the royals for daily Expressen and national broadcaster Sveriges Television (SVT), said earlier on Thursday that she didn't think the news of the engagement came as a total "bolt from the blue".
"At the same time it is pleasant surprise, and I, like everyone else, am now looking forward to an engagement interview. Many are also asking whether the wedding with be broadcast publicly, and much points to that being the case," said von Sydow to TT.
Long time expert on the royals, Sten Hedman, wasn't especially surprised by the news of the princess's engagement either:
"It was pretty much expected. They have been living together after all. It would have been more of a surprise if they hadn't gotten engaged," he said to TT.
Chris O'Neill, who according to Swedish tabloids is a millionaire financial adviser who attended Columbia University, met Princess Madeleine in New York mere months after she broke off her previous engagement to lawyer Jonas Bergström in 2010.
There was wild speculation in the Swedish media at the time of Bergström having had an affair and Madeleine has later said in interview that the months after her engagement to Bergström was broken off were very difficult for her.
"That the whole thing became so public meant that it was a very difficult time in my life," she said in an interview with Dagens Industri.
News of Madeleine and O'Neill's relationship first surfaced in media reports in early 2011 and in February of this year, the Royal Court confirmed that Madeleine and O'Neill had been living together in his Manhattan apartment since November 2011.
O'Neill made his first official appearance with the royal family in Stockholm at the May 2012 baptism of Princess Estelle.
While in New York, the princess has been working with the World Childhood Foundation, a charity for marginalized children set up in 1999 by her mother.
London-born O'Neill, 38, works in finance and is currently a partner at Noster Capital, which has offices in New York and London.
The couple said O'Neill proposed in early October after asking the King's permission.
"It was a very romantic and intimate proposal, but more details about the proposal we want to keep to ourselves," Madeleine said.
They plan to continue living in New York for the time being, she said.
Madeleine's older sister is Crown Princess Victoria, 35, who married Daniel Westling in June 2010. They have a daughter, Princess Estelle, born in February.
The newly-engaged princess also has a brother, Prince Carl Philip, 33.
According to the Royal Court, no date has been set, but the wedding is planned for the summer of 2013.